Introduction
A magnet that measured to spec on the bench can come back from the field 15% weaker — with no visible damage, no cracks, and no corrosion. The part was not defective. It was under-specified. In almost every case, the missing number is coercivity: the magnet's ability to resist an opposing magnetic field without losing its magnetization. Pull force tells you how strong a magnet is today. Coercivity tells you whether it stays that way at 120 °C, next to a stator winding, for eight years.
What Is Coercivity?
Coercivity is the reverse field strength required to demagnetize a permanent magnet. It appears on every datasheet as two separate values, and they are not interchangeable.
Normal coercivity (HcB) is the reverse field needed to drive the magnet's flux density (B) to zero, read off the normal demagnetization curve. At HcB, the magnet appears to have no field at its surface — but its internal magnetization is still intact.
Intrinsic coercivity (HcJ) is the reverse field needed to drive the magnet's magnetization (M or J) to zero, read off the intrinsic curve. At HcJ, the material itself is genuinely demagnetized. HcJ is always the larger of the two, and it is the number that governs demagnetization resistance.
Design rule: specify HcJ. HcB describes an external cancellation effect; HcJ describes the material's actual stability margin.
Think of the magnet as a spring. Compress it moderately and it returns to shape. Push past its yield point and the deformation is permanent. A low-HcJ magnet in a strong reverse field behaves the same way — the loss does not recover when the field is removed.
normal (B–H) and intrinsic (J–H) demagnetization curves
Why Do Magnets Have Coercivity at All?
Coercivity originates in the material's microstructure. A sintered magnet is made of many microscopic magnetic domains, each with its own magnetic moment. Magnetizing the part aligns those moments, producing a net external field. A reverse field, elevated temperature or mechanical shock can flip domains back out of alignment — and coercivity measures how hard that is to do.
Three factors set the value:
|
Factor |
Mechanism |
Effect on HcJ |
| Crystal structure | Magnetocrystalline and shape anisotropy fix the energy barrier to domain reversal | Sets the intrinsic ceiling of the material |
| Defects and internal stress | Grain boundaries and residual stress pin domain walls | Raises HcJ; strongly process-dependent |
| Chemical composition | Heavy rare-earth additions — dysprosium (Dy), terbium (Tb) | Raises HcJ substantially, at a cost in remanence and price |
That last row is why two magnets of the same physical size and similar pull force can differ sharply in price. You are not paying for strength. You are paying for stability.
How Is Coercivity Measured?
Coercivity is measured on a hysteresigraph (also called a permeameter or B-H tracer). The instrument applies a controlled field to the sample and records flux density (B) and magnetization (M) against applied field (H), tracing the full hysteresis loop. From that single measurement you extract remanence (Br), HcB, HcJ and maximum energy product (BHmax).
Units follow two conventions, both in active use:
- SI: kA/m (kiloamperes per metre)
- CGS: Oe (oersted) or kOe — still dominant in North American magnet datasheets
The conversion is 1 kA/m ≈ 12.566 Oe, so 1000 kA/m ≈ 12.6 kOe. Measurement practice is covered by IEC 60404-5 and ASTM A977 — check which standard a supplier's data is reported against before comparing figures across vendors.
hysteresigraph and a measured demagnetization curve
Why Coercivity Matters More Than Pull Force
Most buyers specify a magnet by how strongly it pulls. That number is measured once, at room temperature, in isolation. It says nothing about the three conditions that actually destroy magnets in service:
• Elevated temperature — HcJ falls as temperature rises, typically 0.5–0.7 %/°C for sintered NdFeB. A magnet with comfortable margin at 20 °C can sit past its knee point at 100 °C.
• Opposing fields — in motors, actuators and couplings, the magnet works against fields generated by the windings or by adjacent magnets.
• Mechanical shock and vibration — repeated impact assists domain reversal in low-coercivity materials.
When HcJ is too low for the operating point, the magnet crosses the knee of its demagnetization curve and the loss becomes irreversible. Cooling the assembly back down does not restore performance; only remagnetization does, and that is rarely practical once the part is assembled. In a servo motor, a sensor assembly or a medical actuator, that silent 10 % drift shows up as a control error long before anyone suspects the magnet.
Coercivity by Material: NdFeB, SmCo, Ferrite and AlNiCo
Intrinsic coercivity varies by more than an order of magnitude across the four main permanent magnet families.
Two points that the raw ranges hide:
NdFeB and SmCo overlap at room temperature — and diverge with heat. Top-grade NdFeB out-performs SmCo on the bench. Above roughly 150 °C, SmCo's far smaller coercivity temperature coefficient makes it the more stable choice, which is why it persists in aerospace and downhole applications despite the cost.
AlNiCo's low coercivity is a design constraint, not a defect. It is why AlNiCo magnets are made long and thin, and why they must be magnetized in the assembled circuit — a short, squat AlNiCo magnet largely demagnetizes itself.
Reading NdFeB Grade Codes
Sintered NdFeB grades carry a letter suffix that encodes the guaranteed minimum HcJ — the practical shorthand for "how much heat this part can take."
| Suffix | Typical minimum HcJ | Typical maximum operating temperature* |
| N (none) | ≥ 955 kA/m (12 kOe) | ~80 °C |
| M | ≥ 1114 kA/m (14 kOe) | ~100 °C |
| H | ≥ 1274 kA/m (16 kOe) | ~120 °C |
| SH | ≥ 1353 kA/m (17 kOe) | ~150 °C |
| UH | ≥ 1592 kA/m (20 kOe) | ~180 °C |
| EH | ≥ 1911 kA/m (24 kOe) | ~200 °C |
[Verify: maximum operating temperature is not a material constant. It depends on the magnet's geometry and the surrounding magnetic circuit — specifically its permeance coefficient. Treat the right-hand column as directional guidance only, and confirm the operating point against the actual demagnetization curve at temperature.]
[Verify: grade-code minimum HcJ values follow common industry convention but vary slightly between manufacturers. Confirm against the specific supplier's datasheet.]
How to Specify Coercivity Correctly
1. Define the worst-case operating temperature, not the nominal one. Include self-heating from adjacent windings.
2. Request the demagnetization curve at that temperature — not just the 20 °C curve. The knee position at 120 °C is what matters.
3. Estimate the reverse field the magnet will see from windings or opposing magnets in the assembly.
4. Confirm the operating point sits above the knee, with margin, at the worst-case temperature.
5. Balance against the other three specs: remanence (Br), maximum energy product (BHmax), corrosion resistance and cost. Higher HcJ generally costs remanence and money.
Over-specifying coercivity is expensive — heavy rare earths carry a real price premium. Under-specifying it is worse, because the failure appears only after deployment.
FAQ
Q1: What is the difference between HcB and HcJ?
A: HcB is the reverse field that reduces the magnet's flux density to zero; HcJ is the reverse field that reduces its internal magnetization to zero. HcJ is always the higher value, because at HcB the material remains magnetized even though its external field reads zero. Use HcJ to evaluate demagnetization resistance.
Q2: Does high coercivity mean a stronger magnet?
A: No. Strength is governed by remanence (Br) and maximum energy product (BHmax); coercivity governs stability. In sintered NdFeB the two trade off against each other — raising HcJ with dysprosium or terbium typically reduces Br. A high-coercivity grade is more resistant to demagnetization, not more powerful.
Q3: At what temperature do NdFeB magnets lose their magnetism?
A: There is no single figure. Standard N-grade NdFeB may begin irreversible loss near 80 °C, while EH grades hold to roughly 200 °C. The limit depends on the grade's HcJ and on the magnet's geometry and magnetic circuit, so it must be verified against the demagnetization curve at the actual operating point.
Q4: Is demagnetization reversible?
A: Partly. Loss caused by operating above the knee of the demagnetization curve is irreversible and does not return on cooling — remagnetization is required. Loss that occurs while the operating point stays above the knee is reversible and recovers as the magnet cools.
Q5: When should I choose SmCo over NdFeB?
A: Choose SmCo when the operating temperature exceeds roughly 150 °C, when thermal stability of output matters more than peak field strength, or when corrosion resistance without coating is required. NdFeB delivers higher energy density at lower cost below that threshold.
Post time: Aug-07-2026